The present invention relates to lithographic patterning, and more specifically, to calibrating and aligning layers of lithographic patterns.
Lithographic patterning methods often include patterning a series of layers of lithographic patterns on a substrate. The layers of lithographic patterns are often aligned using alignment marks that are defined in the layers. For example a first lithographic layer having a first set of alignment marks may be patterned on a substrate. A subsequent lithographic layer having a second set of alignment marks is aligned to the first patterned lithographic layer using the first set of alignment marks. Aligning to the first set of marks includes measuring the error in the position of the marks relative to their intended locations (the locations specified in the design) using an alignment metrology tool, which is typically a sub-system of the lithography exposure tool. After aligning to the first set of marks, the second lithographic layer containing the second set of marks is patterned on the substrate. The location of the second set of alignment marks relative to the first, known as “overlay”, may be measured using overlay metrology tools. Overlay errors may be caused by the lithographic equipment or process. The accuracy of the alignment and overlay measurements may be limited by the precision, calibration, and matching capabilities of the metrology tools.